The invention relates to electrolytic cells for electrowinning metals from fused salt baths, especially aluminium from a fused cryolite-alumina bath. In the conventional Hall-Heroult process for aluminium electrowinning, consumption of the carbon anodes entails significant costs. The possibility of using metal oxides as anodes instead of consumable carbon anodes was investigated by A. I. Belyaev more than forty years ago (see, e.g., Chem. Abstr. 31, 1937, 8384 and 32, 1938, 6553). The state of the art relating to metal oxide anodes proposed for aluminium electrowinning may be illustrated for example by U.S. Pat. Nos. 4,039,401, 4,057,480, 4,098,669, 4,146,438, 3,718,550.
The use of inconsumable anodes for aluminium electrowinning would eliminate the significant costs of carbon replacement required for the carbon anodes currently used, as well as emissions from the cell, while allowing closer control of the anode-cathode gap. On the other hand, the oxygen evolution potential on an inconsumable anode would be higher than for the evolution of CO.sub.2 on the carbon anode. The electrical energy consumption for aluminium production would thus be increased accordingly, unless other modifications are made in the design and mode of operation of the electrolytic cell.
The development of inconsumable anodes for aluminium electrowinning from fused cryolite-alumina is particularly difficult due to the fact that they must meet extremely strict requirements with regard to stability and conductivity under severe operating conditions. Such anodes must firstly be substantially insoluble and able to resist attack by both the cryolite-alumina bath at high temperature (about 1000.degree. C.) and anodically generated oxygen. This first requirement is essential since contamination of the molten aluminium recovered at the cathode above the tolerated impurity levels would be undesirable.
In addition, inconsumable anodes having a higher electrical resistivity than the cryolite-alumina bath (about 0.3 ohm.cm) would have an uneven current distribution, whereby the anode current density may increase considerably towards the surface of the bath. Further, uneven distribution of the current density along the anode is also undesirable since it may contribute to corrosion of the anode near the phase boundary between the molten salt bath and the surrounding atmosphere (see e.g. U.S. Pat. No. 4,057,480).
Thus, for the reasons already mentioned, the electronic conductivity of the anode should be greater than 4 ohm.sup.-1 cm.sup.-1 at 1000.degree. C. Pure non noble metals have high conductivity but are unstable as anodes in fused cryolite-alumina. On the other hand the use of noble metals having adequate stability is restricted by their high cost. Further, the metal oxides which have been proposed as anode materials generally have inadequate electronic conductivity.